1. Field of the Invention
The invention relates to process instrumentation and, more particularly, to a field device for process instrumentation, which has a standardized 4-20 mA interface for outputting measured values.
2. Description of the Related Art
Versatile field devices are used for process instrumentation for controlling processes in process installations. Measurement transducers are used to detect process variables, such as temperature, pressure, flow rate, filling level, density or gas concentration of a medium. The process sequence can be influenced by actuating elements, as a function of detected process variables, corresponding to a strategy which, for example, is predetermined by a control station. A control valve, a heater or a pump are examples of actuating elements. Particularly in process installations, pressure measurement transducers represent major sensor components for the purposes of automated production sequences. High-quality measurement transducers, which produce disturbance-free measured values, with little susceptibility to errors or faults, even in extreme conditions, are required for an optimum installation behavior and high product quality all the time.
Siemens Catalogue “ST FI 01-2008”, Chapter 1 discloses a pressure measurement transducer for process instrumentation, which is equipped with a standardized 4-20 mA interface for outputting a measured value, for example, to a control system. The parameterization is performed digitally using the HART protocol. The measured values of the pressure to be measured are in contrast transmitted as an analogue current.
FIG. 1 shows a conventional design of the output stage for production of the analogue current signal in the 4-20 mA interface of a conventional pressure measurement transducer. A two-wire line, which is not illustrated in the figure, can be connected to two connecting terminals 1 and 2, by which two-wire line the measurement transducer in a process installation can be connected, for example, to a controller station or to an automation appliance. In the output stage, a loop current I is passed via a series circuit that consists of a transistor circuit 3 of an electrical power supply 4 and a measurement resistor 5. The transistor stage 3 is operated by a regulator 6 with a control signal 7.
The loop current I, which is adjusted by the transistor circuit 3, therefore depends on the magnitude of the control signal 7. The electrical power supply 4 supplies the measurement transducer with the power required for its operation. The available power is governed by the magnitude of the loop current I and the voltage U_IN that is dropped across the electrical power supply 4. A portion of the loop current I is supplied through the electrical power supply 4 to a downstream DC/DC converter 8, which uses this to produce voltages 9 that are suitable for operation of the electronic circuits in the measurement transducer. These are not illustrated in FIG. 1, for the sake of clarity.
The high-precision measurement resistor 5 is used to detect the loop current I, and converts this to a voltage signal, which is supplied as an actual value U_ACT to the regulator 6 in a first feedback path 10. The regulator 6 ensures that the detected actual value U_ACT is regulated at a nominal value U_NOM, which is generated, corresponding to the measured pressure, by a microcontroller, which is not illustrated in FIG. 1. The regulator 6 is a regulator with an integrated behavior, which therefore forms a control error from the nominal value U_NOM and the actual value U_ACT, and integrates this control error. The symbol for the regulator 6 illustrated in FIG. 1 shows only its central element, an operational amplifier, for illustrative purposes.
The measurement transducers, in particular, are subject to ever more stringent requirements for measurement accuracy and functional scope. The increasing requirements also have effects on the design of the output stage of the 4-20 mA interface. The requirements relating to the accuracy of the output measured value are, for example, influenced directly by the setting accuracy of the loop current I with the aid of the measurement resistor 5, the regulator 6 and the transistor circuit 3. An operational amplifier that is highly precise and has a low offset voltage is required in particular for the implementation of the regulator 6. Furthermore, the output stage of the interface has to manage with a very low current draw in order that sufficient operating power is available to provide the desired functional scope for the other electronic circuits of the measurement transducer. Particularly with regard to the choice of the operational amplifier and transistor circuit, this means that only components that draw very little current may be used. Furthermore, the output circuit must be distinguished by a high level of EMC (electromagnetic compatibility) or immunity to noise. This means the capability of the output stage to withstand disturbance variables of a specific level without malfunction. According to Criterion A of NAMUR Recommendation NE21, no identifiable adverse effect on the function should be caused by specific disturbances. For example, NE21 specifies as a disturbance radio-frequency injection in the frequency range from 10 kHz to 80 MHz, and an amplitude of 10 V, or application of rapid transients (burst) at 1 kV via the two-wire line. The current output of the field device with a 4-20 mA interface and a class accuracy of 0.5% must not differ from the nominal value by more than 0.16 mA under a disturbance influence, when the actual output value is 12 mA.
In terms of the stated requirements, the known output stage has the following disadvantages:
The gain of the transistor circuit 3 is limited, because only a single PNP power transistor is used, which has a gain in the order of magnitude of 50. This requires a relatively high drive current for the transistor, which flows away to ground GND. The drive current is therefore no longer available for producing the power required for operating the measurement transducer. The power available for the electronics is therefore reduced by about 3%.
In order to compensate for disturbances that are injected into the output stage via a two-wire line connected to the connections 1 and 2, the regulator 6 must have, in addition to the required accuracy for the adjustment of the loop current I, a high operational speed. This can be achieved only by an operational amplifier that has a comparatively high power consumption. In comparison to the use of an ultra-low-power version, this results in a reduction in the power available for supplying the electronics by a further 3%.
6% of the available operating power is therefore actually consumed in the known output stage. With the continuously more stringent requirements for field devices in terms of their performance with respect to measurement rate, functional scope, graphics display, display lighting, SIL (Safety Integrity Level) in accordance with IEC/EN61508, etc., it is becoming increasingly necessary for the electrical power, which is urgently required to supply the electronics for providing the large number of functions, to no longer be lost to this extent in the output stage.